Heat tolerance classification criteria require population‐specific thresholds for accurate assessment of acclimation state in adults

Abstract Heat tolerance testing (HTT) assesses responses to heat stress with rectal temperature (T rec ) and heart rate (HR) thresholds defining individuals as heat‐tolerant (HT) or heat‐intolerant (HI). To evaluate classification criteria by acclimation state and sex. Forty participants (19M/21F, mean ± SD, 23 ± 4 years) completed an HTT (120 min, 5 km·h −1 , 2% grade) before (PreHA) and after (PostHA) 5 days of repeated exercise in (40°C, 40% RH) categorized as isothermal (exercise intensity adjusted to maintain T rec within 38.5°C–39.5°C, 60 min) exercise‐heat acclimation (HA). Females had lower body surface area (vs. males) (1.7 ± 0.1 vs. 2.0 ± 0.2 m 2 , p < 0.001), mass (59.5 ± 5.4 vs. 79.5 ± 10.1 kg, p < 0.001), height (164.2 ± 5.7 vs. 179.1 ± 7.6 cm, p < 0.001), maximal oxygen consumption (V̇O 2 max, 44.5 ± 5.1 vs. 51.5 ± 5.7 mL·min −1 ·kg −1 , p < 0.001), and velocity at V̇O 2 max (12.9 ± 1.5 vs. 15.0 ± 1.7 km·h −1 , p < 0.001). PreHA 51.1% (mean % HI classification across criteria) of participants were HI (66.7% F/52.6% M), decreasing ( p < 0.001) to 23.6% PostHA (28.6% F/10.5% M), reflected across criteria ( p < 0.05). The rate of HI based on plateau in T rec during the final 60 min (ΔT rec (T120–T60) ≥ 0.45°C) was similar ( p = 0.735) PreHA (15% HI) and PostHA (10%), and HA reduced ΔT rec (T120–T60) (0.30 ± 0.19 vs. 0.19 ± 0.20°C, p = 0.001), demonstrating sensitivity to adaptation. HA reduces HI classification rate, but criteria differ in capturing presumed HT and classify more females as HI in naïve and acclimated states.